Existing wind turbine blades are generally manufactured from reinforced composite materials. A typical blade is fabricated in two shells, which are subsequently united to form a single unit. The shells include at particular location sandwich panel regions having a core of lightweight material such as foam or balsa wood.
Different regions of a wind turbine blade are subject to different forces. Consequently, the thickness of the core generally varies across the blade for structural reasons. Typically, the core thickness ranges from 5 mm to 45 mm.
A prior art core 10 is shown schematically in FIG. 1a. Referring to FIG. 1a, the core 10 includes several parallel slits 12, which facilitate draping of the core 10 so that the core 10 may conform to the curvature of the blade shell, as shown schematically in FIG. 1b. Different regions of a blade have different curvatures. Consequently, the core 10 may be required to drape to different extents in different regions of the blade.
It is desirable to introduce radar absorbing material (RAM) into the composite structure of blades. One reason for this is that rotating blades have a radar signature similar to that of aircraft, which can make it difficult for air traffic control and other radar operators to distinguish between aircraft and wind turbines. Incorporating RAM into blades ensures that the resulting blades have a reduced radar signature that can be distinguished easily from aircraft, and which creates less unwanted events (also known as “clutter”) on the screen of the radar operator.
FIG. 1b shows a known technique for incorporating RAM into a blade. Referring to FIG. 1b, a blade 14 includes a radar absorbing layer 16 close to its outer surface. The drapable core 10 of FIG. 1a is provided inboard of the radar absorbing layer 16, and a radar reflecting layer 18 is disposed beneath the core 10. The RAM may be a “circuit analogue” (CA) absorber in which the radar absorbing layer 16 comprises a circuit provided on a suitable substrate, for example a glass-fibre cloth, and the radar reflecting layer 18 may suitably comprise a carbon cloth.
The separation between the radar absorbing layer 16 and the radar reflecting layer 18 is a key parameter for absorption performance, and must be carefully controlled to achieve a blade having the desired absorption properties. Such careful control of the separation of layers is made more difficult by varying geometry of the blade, specifically the abovementioned variation in core thickness.